The maturation of the microsomal enzyme, Bilirubin-UDP-glucuronyl transferase of liver is to be quantitatively measured in human fetuses and newborn infants. The gestational influences upon the maturation of this enzyme will be studied in experimental rats by induction of post maturity. The products of the in-vitro and in-vivo conjugation of bilirubin will be determined. The magnitude of the enterohepatic circulation by bilirubin in neonatal hyperbilirubinemia will be determined by blocking reabsorption of bilirubin with agar in both experimental rats and jaundiced infants. The mechanism by which agars bind bilirubin will be studied by determining its binding characteristics and environmental requirements. By sequestering bilirubin within the intestine in jaundiced Gunn rats it will be possible to study the protection of the animals from bilirubin encephalopathy and bilirubin nephropathy. The protection of the kidney can be assessed from the osmolar and free water clearances in vivo. The Na-K-ATPase, osmolality and solute composition of the renal medulla will be determined at the time of sacrifice. The photo-oxidation of bilirubin in blood will be studied both in-vivo and in-vitro. Examination for other photo-oxidations will be performed by measurements of aromatic amino acid concentrations in serum and in serum albumin. Photo-oxidative injury to erythrocytes will be determined from changes in membrane ATPase activity and hemoglobin oxygen dissociation curves.